Conventional axial-flow blood pumps with hydrodynamic bearings used in cardiac assist, such as the pump disclosed in U.S. Pat. No. 4,525,712, required a supply of purge fluid to prevent blood from entering their hydrodynamic journal and thrust bearings and causing thrombus formation, hemolysis and bearing seizure. Because of this need for an external fluid supply, that type of pump is not well suited for long-term implants.
Ideally, implantable blood pumps should require no bearing fluid or else use the pumped blood itself, or components of the pumped blood, as a bearing fluid. Indeed, constructions which allow this have been proposed, among others, by R. K. Jarvik in U.S. Pat. No. 4,994,078 and by Isaacson et al. in U.S. Pat. No. 5,112,200. The problem with these constructions is that they rely on cylindrical radial or journal bearings which mechanically support the rotor against radial movement. In typical embodiments of the prior art, those bearings are interior film bearings, i.e. blood-lubricated cylindrical hydrodynamic bearings through which blood serum is drawn by the pressure differential between the ends of the cylinder. In order to prevent blood cells from entering the bearing and being hemolyized, the bearing clearance is made so small that blood cells are essentially precluded from entering the bearing.
Alternatively, as taught by U.S. Pat. No. 4,704,121 to Moise, bearing fluid for a magnetically driven blood pump can be obtained by filtering a portion of the pumped blood through a filter which retains the blood cells and proteins but passes the serum.
Papers entitled "Axial Flow Ventricular Assist Device: System Performance Considerations" (Artificial Organs, Vol. 18, No. 1 pp., 44-48 (1994) and "An Ultimate, Compact, Seal-less Centrifugal Ventricular Assist Device: Baylor C-Gyro Pump" (Artificial Organs, Vol. 18, No. 1, pp. 17-24 (1994) describe, respectively, an axial-flow blood pump and a centrifugal blood pump using blood-lubricated pivot bearings.
The journal or radial bearing concepts of the prior art have a potential flaw which puts them at risk to bearing seizure, and/or ultimately causes them to undergo excessive bearing material wear. Fundamentally, this is due to the length of the bearing, the diminished heat removal capacity caused by the location of the bearings inside the rotor or stator, and to the lack of significant bearing through-flow in the interior film designs. The comparatively long and extremely narrow gaps the blood must pass through are subject to be plugged by denatured blood products. This is particularly true in those prior art embodiments in which the journal bearing is closed at one end, so that blood cannot flow through it. Even in those designs in which the motor has sufficient torque to machine through any residue formed, significant material wear can occur over the long term and reduce the pump's useful life. Also, in journal bearings using the extremely small tolerances necessary to prevent entry of blood cells, the slightest misalignment of the rotor with respect to the stator can seriously impair the functioning and the life of the pump. Finally, the performance and longevity of journal bearings, including interior film blood bearings, are significantly more dependent on difficult-to-control patient variables such as blood chemistry and hemorrhology than arrangements which do not use blood lubrication.
A need therefore exists for an implantable blood pump in which blood lubrication is unnecessary, alignment is not critical, the interface area between rotating and stationary elements is kept very small, and the interface has superior heat-removing ability and resists any shape changes due to wear.